1. Field of the Invention
The present invention relates to a rotary connection unit for use in electrically connecting two parts relatively rotating in automobiles, home electric equipment and the like.
2. Description of the Related Art
In such an automotive steering wheel mount as shown in FIGS. 7 and 8, for example, a steering shaft 2 is rotatably supported with a bracket 1 fixed to a body, whereas a switch unit 3 and a rotary connection unit 4 are installed on the surface of the bracket 1. Moreover, a steering wheel 6 rotating together with the shaft 2 is fitted to the fore-end of the steering shaft 2 passed through these switch unit 3 and the rotary connection unit 4 and projected upward. Furthermore, the steering wheel 6 is provided with electric equipment 7 such as a horn switch and an air bag as occasion demands. The switch unit 3 incorporates, for example, a turn signal lamp switch, a wiper switch and the like, operating levers 8, 9 for turning on and off the switches being each fitted to the left and right sides of the switch unit 3.
The aforesaid rotary connection unit 4 is a unit for electrically connecting the electric equipment 7 making a rotary movement together with the steering wheel 6 to a circuit on the body side and has heretofore been so configured as shown below by way of example.
In a first conventional rotary connection unit shown in FIGS. 9A-11, a cable (flexible flat cable; FFC) 13 is contained in an annular space 12 between a fixed-side member 10 fixed to the body side and a rotary-side member 11 rotating together with the steering wheel 6 in such a manner that the cable is spirally wound on the rotary-side member 11; a connector 15 fitted to the outer end portion of the cable 13 is connected to a connector 16 installed on the side of the bracket 1 as shown in FIG. 8; and a connector 17 fitted to the inner end portion of the cable 13 is connected to a connector 18 provided to the electric equipment 7. As shown in FIG. 9A, the cable 13 is wound around the outer peripheral portion of the rotary-side member 11 when the rotary-side member 11 is rotated clockwise and as shown in FIG. 9B, whereas the cable 13 is rewound in such a manner as to creep along the side wall of the fixed-side member 10 when the rotary-side member 11 is rotated counterclockwise. Thus, an electric circuit on the body side and the electric equipment 7 of the steering wheel 6 are kept electrically contacting each other even though the fixed-side member 10 and the rotary-side member 11 are relatively rotated.
As shown in FIGS. 10A and 10B, therefore, there are some rotary connection units having a rotary guide 20 for reversing the direction of winding the cable 13 midway so as to make movable the reversing portion in conjunction with the rotation of the rotary-side member 11. This arrangement is contrary to what is shown in FIG. 9 in that the cable 13 is rewound in such a manner as to creep along the side wall of the fixed-side member 10 when the rotary-side member 11 is rotated clockwise as shown in FIG. 10A, whereas the cable 13 is round around the outer peripheral portion of the rotary-side member 11 when the rotary-side member 11 is rotated counterclockwise as shown in FIG. 10B. With this arrangement, the whole length of the cable 13 can be made shorter than that of the cable 13 shown in FIG. 9 by reversing the direction of winding the cable 13 midway.
As shown in FIG. 11, the cable 13 wound around the central axis of rotation is made flat by arranging a plurality of conductors in the direction of a central rotational axis 19, that is, in the direction of a vertical rotational axis. Therefore, the width in the direction of the central rotational axis 19 of the cable 13 becomes greater when the number of conductors of the cable 13 is increased and in the case of a rotary connection unit for an automotive steering wheel mount, it becomes difficult to secure a space in which the rotary connection unit is assembled. Moreover, the troublesome step of winding the cable 13 is needed at the time of assembly and though the end portion of the conductor of the cable 13 has heretofore been connected directly to an electric circuit or indirectly via connectors thereto at the time assembly, these sorts of work are also laborious and result in necessitating hours to do assembling work.
Although the use of a flexible print circuit (FPC) in place of the aforesaid FFC as the cable 13 is considered feasible, the problem is that the whole length of the cable 13 may become as long as approximately two meters, thus making the manufacture of a cable of this sort extremely difficult.
In the case of a second conventional rotary connection unit shown in FIGS. 12 and 13, on the other hand, a plurality of conductors are lined up in parallel within a plane intersecting the aforesaid central rotational axis 19 in the cable 13 (FFC or FPC) and the cable 13 is spirally formed in parallel to the well-ordering plane of the conductors. Moreover, the cable 13 is folded back in the intermediate portion of the rotary guide 20 so that its spiral direction is reversed. In this case, one end portion 13a of the cable 13 maybe held by the fixed-side member 10 and the other end portion 13b is held by the rotary-side member 11 (see Japanese Patent Unexamined Publication No. 215071/1990).
Since the width in the direction of the central rotational axis is unchanging even though the number of conductors of the cable 13 is increased in the case of the second conventional example, the size of the rotary connection unit 4 in the direction of the central rotational axis is restrained from being increased and the work of spirally winding the cable 13 can be dispensed with. This is advantageous as the coils of the cable are prevented from becoming loosened.
However, the second conventional example poses a problem in that though the size of the rotary connection unit 4 in the direction of the central rotational axis 19 remains unchanging, its size in a direction perpendicular to the central rotational axis 19 conversely tends to increase.